Patterns of doramectin tissue residue depletion in parasitized vs nonparasitized lambs.

The effect of gastrointestinal parasitism on patterns of edible tissue depletion of doramectin was studied in greyface Suffolk lambs. Twelve weight-matched pairs of lambs were allocated into group I (nonparasitized, pretreated with three administrations of 5 mg/kg fenbendazole) and group II (parasitized, did not receive anthelmintic treatment). Both groups were maintained together under similar conditions for 70 days, when they were treated with a subcutaneous dose of 0.2 mg/kg bw doramectin. At 7, 14, 21, and 28 days after doramectin administration, three lambs from each group were slaughtered and samples of liver, kidney, muscle, and fat were obtained. Pre-treatment with fenbendazole significantly reduced the nematode fecal egg count and significantly increased lamb body weight compared to the parasitized group. Doramectin was detected in all of the tissues up to 28 days post-treatment. Significantly higher and more persistent doramectin concentrations were found in the nonparasitized lambs compared to the parasitized animals. Considering the EMEA maximum residue limits for doramectin in fat, the calculated withdrawal period for the healthy lambs (43 days) was significantly higher than that for the parasitized animals (26 days).